In the precharacterizing clause, the invention refers to a prior art for isolation apparatuses as is described, by way of example, in U.S. Pat. No. 5,434,550 A. One of the described isolation apparatuses is used for disconnection of a surge arrester from ground potential and has a dielectric material enclosure, out of which two power connections are passed. A cartridge filled with an explosive substance as well as two current paths that are connected in parallel between the two power connections are provided in the interior of the enclosure. One of the two current paths that has the lower impedance contains a fuse wire that is wound in the form of a coil, while one of the two current paths which in contrast has a high impedance contains a spark gap arranged between two rings. During normal operation of the surge arrester, only a very small leakage current flows through the fuse wire, which is sufficient only to heat the isolation apparatus to a permissible extent. When a brief overvoltage occurs in a network that contains the surge arrester, for example as a result of a switching operation or as a result of a lightning strike, a high current flows through the isolation apparatus for a short time. This current is commutated into the current path that contains the spark gap, with an arc being formed. If the overvoltage lasts for a relatively long time, however, owing to a faulty state in the network, then the fuse wire is melted and an arc is formed in the enclosure interior, which activates the cartridge. This results in the enclosure being detonated, and the surge arrester is disconnected from ground potential. The same effect is achieved if a faulty state of the surge arrester results in an excessively high leakage current being carried in the fuse wire. This current gradually heats the isolation apparatus to such an extent that, above a limit temperature, the cartridge is activated and the connection is disconnected as a result of the explosion. When the explosion occurs, an indicating element is made visible, and the current is commutated into a grounding conductor which contains the indicating element. However, the explosive substance in the cartridge of the isolation apparatus represents an undesirable potential hazard.
An isolation apparatus of the type mentioned initially for a surge arrester is also described in DE 100 30 669 A1. In this apparatus, a gas generator based on an airbag is arranged in a blind hole in a conductor section of a dissipation current path. When a long-lasting fault current occurs, the airbag is fired by an electrical signal which is produced in an inductive transformer through which the fault current flows. An apparatus such as this is comparatively complex.